Fix fake LINT check success (#8092)

The code 'if [ -z ${something} ]; then ... fi' means "if
${something} is an empty string, yell at the command line
about 'binary operator expected' and ignore the body of the
if statement, if ${something} is not an empty string,
the condition is false so ignore the body of the if
statement" which clearly isn't what the author wanted. Fix
it by adding a few quotes around the offending ${something}.

util/travis/lint.sh

@@ -20,7 +20,7 @@ function perform_lint() {
 			whitelisted=$(awk '$1 == "'$f'" { print 1 }' "$CLANG_FORMAT_WHITELIST")
 
 			# If file is not whitelisted, mark a failure
-			if [ -z ${whitelisted} ]; then
+			if [ -z "${whitelisted}" ]; then
 				errorcount=$((errorcount+1))
 
 				printf "The file %s is not compliant with the coding style" "$f"